The signals that are to be detected are composed of at least one frequency chosen from a finite set of frequencies.
More particularly this invention will deal with multifrequency signals that are usually composed of a combination of two pure frequency signals. These signals can, for example, be the dialing signals of a telephone keyboard. In this case, depressing any one of the keys of the keyboard causes a multifrequency signal, corresponding to the combination of two frequencies chosen in a set of eight predetermined frequencies, to be transmitted to the central office (PBX or CX). The identification of the depressed key, and hence recognition of the requested telephone number, is performed in the central office by a so-called multifrequency receiver (MFR) whose task is to detect the presence of the multifrequency signal in the received wave and to identify the two frequencies forming the multifrequency signal received.
In general, one of the two frequencies coupled to form a multifrequency signal belongs to a so-called low-frequency band (about 700 to 1000 Hz for example), the other one belongs to a so-called high-frequency band (about 1200 to 1700 Hz for example). Thus, the simplest MFR receiver could consist of two filters only, i.e. a low-pass filter (LPF) and a high-pass filter (HPF), both cutting off between 1000 and 1200 Hz. Each of the bands thus obtained would in turn be split into four subbands, each one being defined so as to include only one of the eight predetermined frequencies mentioned above. The MFR problem is thus reduced to detecting two pure frequency signals (tone), one being in the low band (the LPF filter band), the other being in the high band (the HPF filter band). The MFR receiver is thus composed of two similar tone receivers, one for detecting and identifying a high frequency, the other one a low frequency. One might thus assume that the MFR function may be then completed by measuring the energy in each of said subbands and by deriving therefrom an identification of the subbands carrying the highest energy within each of the two frequency bands, i.e. high and low.
However, proceeding in this manner without taking additional precautions would provide a circuit that is particulary sensitive to erroneous detections. It would not be capable of distinguishing a real numbering signal from any ambiant noise, e.g. a speech signal.
An improvement to the above MFR has been proposed which includes a limiter circuit at the output of the LPF and HPF filters. Such a system has been described in the Bell System Technical Journal (BST) of September 1981, volume 60, No 7, pages 1574-1576. The limiter circuit plays a double part: first, it acts as an amplitude limiter to normalize the amplitude of the output signals of the LPF and HPF filters and second, it tends to favor the subband signals containing a pure frequency (case of the signals to be detected by the MFR) over those containing several frequencies (noise).
It should be further noted that in practice all operations of the MFR receiver are performed in digital mode. This means that before the signal is presented to the LPF and HPF filters, it is sampled at the Nyquist frequency of about 8 KHz and digitally coded with 12 or 16 bits. All filtering operations executed by the MFR are thus performed in digital mode by means of microprocessors. The use of microprocessors causes a computing noise which can disturb the operation of the MFR receiver. Furthermore, the computing power required for processing 8000 digital samples per second is rather significant and this imparts the MFR cost.